The present invention relates to a radio communication system, a radio communication device, and a radio communication method using microwaves in a specific frequency band, and particularly to a radio communication system, a radio communication device, and a radio communication method for realizing communication operation with low power consumption between devices at a relatively short distance. More particularly, the present invention relates to a radio communication system, a radio communication device, and a radio communication method that perform data communication by a backscatter system using transmission of an unmodulated carrier from a reading device side, and absorption and reflection of the received radio wave on the basis of an operation of terminating an antenna on a transmitting device side, and particularly to a radio communication system, a radio communication device, and a radio communication method that eliminate the effects of transmitter noise on the reading device side to improve reception sensitivity and increase the communication distance.
One example of radio communication means applicable in only a limited area is RFID. RFID is a system including a tag and a reader in which the reader reads information stored in the tag in a non-contact manner. While the system is also referred to as an “ID system,” a “data carrier system” and the like, a universally common name for the system is an RFID system. The RFID system may be abbreviated as RFID. Incidentally, the RFID system is an “identification system using high frequencies (radio waves).” Methods of communication between a tag and a reader/writer include, for example, an electromagnetic coupling type, an electromagnetic induction type, and a radio frequency communication type (see, for example, Klaus Finkenzeller (Translated from the 3rd German edition by Rachel Waddington, Swadlincote, UK) “Fundamentals and Applications in Contactless Smart Cards and Identification” (Wiley & Sons LTD)).
An RFID tag is a device including unique identifying information, and has an operating characteristic of oscillating a radio wave at a modulation frequency corresponding to the identifying information in response to reception of a radio wave of a specific frequency. On the basis of the oscillation frequency of the RFID tag, a reading device side can identify the RFID tag. Hence, a system using RFID makes it possible to identify an article, an owner, and the like using a unique ID written in the RFID tag. The RFID system is now used in many systems including, for example, a system for monitoring the entering and leaving of a room, an article identifying system in distribution, a bill payment system in restaurants and the like, and a system for preventing takeout before payment in stores selling CDs, software and the like.
A radio identification device of a small size can be produced by packaging an IC chip having a transmission and reception function and a memory function, a source for driving the chip, and an antenna (see for example, Japanese Patent Laid-Open No. Hei 6-123773). According to this radio identification device, it is possible to transmit various data on an article or the like to receiving means of the IC chip via the antenna, to store the output in a memory, to read the data in the memory and to supply the data to the outside by radio via the antenna as required. Hence, the presence and position of the article or the like can be checked and traced quickly and easily.
An RFID system includes an RFID tag and a tag reader. When the tag receives an unmodulated wave fO transmitted from the tag reader, the unmodulated wave fO is rectified and converted into direct-current power, and the direct-current power can be used as operating power of the tag. The tag side performs an operation of terminating an antenna according to a bit image of transmission data, and thus uses absorption and reflection of the received radio wave to represent the data. Specifically, when the data is 1, the tag terminates the antenna by an antenna impedance to absorb the radio wave from the tag reader. When the data is 0, the tag reflects the radio wave from the tag reader by setting a terminal of the antenna in an open state. A signal of the same frequency as that of the signal transmitted from the tag reader is returned by the reflection of a backscatter system. A communication method of representing data by a pattern of absorption and reflection of the thus arrived radio wave is referred to as a “backscatter system.” Thus, the tag can transmit information therewithin to the reader side without a power supply.
Conventionally, a radio communication system of the backscatter system is limited in communication range to a relatively short distance, and is thus often applied to identification and authentication of an article, a person and the like, as is typified by the RFID tag.
On the other hand, the RFID tag generally has no power supply, and is supplied with power from the radio wave from the reader. This power is supplied from a battery within the device, whereby radio data transmission with low power consumption by the backscatter system can be realized. That is, when the communication distance is limited, radio communication of the backscatter system has a characteristic of being able to establish a radio transmission line with a very low power consumption. Recently, with improvements in packaging technology, IC chips having a memory function have appeared, and also the memories of the IC chips have been increasing in capacity. There is hence a desire to not only communicate relatively short data, such as identifying and authenticating information, but also to adopt the communication of the backscatter system for general data transmission. For example, the communication of the backscatter system is useful in transmitting images from a digital camera or a portable telephone to a PC, a printer, a TV or the like.
The communication system based on the backscatter system performs data communication using absorption and reflection of the received radio wave on the basis of the operation of terminating the antenna as a fundamental operation. Generally, the frequency of a carrier from the reader and the center frequency of the reflected wave are the same, and the reader side performs transmission and reception at the same frequency.
In such a case, a receiving unit is affected by the transmission frequency that goes around into the receiving unit from a transmitting side, and needs to process the reflected wave having a weak power. That is, the receiving unit is easily affected by a DC offset and transmitter noise, thus making it difficult to increase the transmission distance. In addition, a modulation system in the backscatter system is generally an ASK modulation system or a PSK modulation system in most cases, thus making it difficult to increase speed.
FIG. 7 shows an example of the configuration of a radio communication system of a conventional backscatter system.
Reference numeral 500 denotes a radio transmission device on a mobile device side. Reference numeral 510 denotes a radio transmission and reception device on a reader side. Suppose that data transmission is performed by the backscatter system from the radio transmission device 500 to the radio transmission and reception device 510.
The radio transmission device 500 is connected to an application unit 503, such as a digital camera or the like. Similarly, the radio transmission and reception device 510 is connected to an application unit 519, such as a printer or the like.
The radio transmission and reception device 510 includes an antenna 511, a circulator 512 for separating a transmitted wave and a received wave from each other, a receiving unit 514, a local oscillator 513 shared for transmission and reception by the receiving unit 514 and a transmitting unit 517, and a baseband processing unit 518. Suppose in the example shown in the figure that the receiving unit 514 and the transmitting unit 517 both use a direct conversion system. Further, the receiving unit 514 includes a quadrature demodulation unit 515 and an AGC amplifier 516. An unmodulated carrier is transmitted to the radio transmission device 500 by turning on the transmitting unit 517 by the baseband processing unit 518 and thereby transmitting frequency fO of the local oscillator 513 from the antenna 511 via the circulator 512.
The transmitted unmodulated carrier fO reaches the radio transmission device 500. The radio transmission device 500 includes an antenna 501 and a backscatter modulator 502. The backscatter modulator 502 performs backscatter ASK, PSK, or QPSK modulation according to transmission data of the application unit 503. The modulation can be easily performed by on/off operation of a diode, a GaAs switch or the like. Thus, a modulated wave eventually reflected from the antenna 501 is generated with the center frequency fO of the unmodulated carrier as a center.
In the radio transmission and reception device 510, the backscattered modulated wave having the center frequency fO is received by the antenna 511, the circulator 512, and the receiving unit 514. The quadrature demodulation unit 515 is supplied with the frequency fO of the local oscillator 513, performs direct conversion reception, and generates an I′ signal and a Q′ signal of a baseband signal.
The I′ signal and the Q′ signal of the baseband signal are amplified to a desired level by the AGC amplifier 516 in a succeeding stage. Thereby, an I signal and a Q signal of the baseband signal are obtained. The I signal and the Q signal of the baseband signal are supplied to the baseband processing unit 518. The baseband processing unit 518 performs demodulation, and then supplies received data and a received clock to the application unit 519.
The unmodulated carrier fO from the transmitting unit 517 is emitted from the antenna 511 via the circulator 512, and also goes around into the receiving unit 514 side. This component going around into the receiving unit 514 side can be reduced to a degree by the circulator 512. However, the value of the reduction is not infinite, and an isolation of about 20 dB is an actual value.
FIG. 7 also shows a frequency spectrum on the reader side. Reference numeral 520 denotes the frequency spectrum at an input terminal of the quadrature demodulation unit 515. Reference numeral 521 denotes the modulated wave reflected by backscatter, for example, a BPSK modulated wave. Reference numeral 522 denotes the unmodulated carrier. When the modulated signal 521 is small, the unmodulated carrier 522 has a larger value.
This unmodulated carrier fO enters the quadrature demodulation unit 515 to be mixed with the local frequency fO of the local oscillator 513. Consequently, a high direct-current voltage is generated. This forms a DC offset, which produces a great adverse effect on operation of the quadrature demodulation unit 515. Thus, the very small modulated signal becomes distorted and difficult to demodulate, thereby constituting a major impediment to increasing the transmission distance.
As one method for solving such a problem, there is a method of shifting the reception frequency fO by a predetermined center frequency fS in either a positive direction or a negative direction, and returning a reflected wave on the tag side. In this case, the frequency of the reflected wave received on the tag reader side is not the same as the transmission frequency. Therefore, effects of a DC offset and transmitter noise are avoided, so that the reflected wave can be received with a high sensitivity. Thus, the transmission distance can be increased.
For example, a method of first performing QPSK modulation using a subcarrier and then performing ASK or PSK modulation by the backscatter system as secondary modulation has been proposed (see, for example, Japanese Patent Laid-Open No. Hei 10-209914).
FIG. 5 shows an example of the configuration of an RFID system in which the tag side shifts the reception frequency fO by a predetermined center frequency fS in either a positive direction or a negative direction, and returns a reflected wave.
Reference numeral 100 denotes a radio transmission device on a mobile device side. Reference numeral 110 denotes a radio transmission and reception device on a reader side. Suppose that data transmission is performed by the backscatter system from the radio transmission device 100 to the radio transmission and reception device 110. The radio transmission device 100 is connected to an application unit 105, such as a digital camera or the like. Similarly, the radio transmission and reception device 110 is connected to an application unit 119, such as a printer or the like.
The radio transmission and reception device 110 includes an antenna 111, a circulator 112 for separating a transmitted wave and a received wave from each other, a receiving unit 114, a local oscillator 115 for the receiving unit 114, a transmitting unit 116, a local oscillator 117 for the transmitting unit 116, and a baseband processing unit 118. Suppose in this case that the receiving unit 114 and the transmitting unit 116 both use a direct conversion system.
An unmodulated carrier is transmitted to the radio transmission device 100 by turning on the transmitting unit 116 by the baseband processing unit 118 and transmitting frequency fO of the local oscillator 117 from the antenna 111 via a band-pass filter 113 and the circulator 112. The transmitted unmodulated carrier fO reaches the radio transmission device 100. The band-pass filter 113 is provided to reduce the effects of transmitter noise on the receiving unit 114.
The radio transmission device 100 includes an antenna 101, a backscatter modulator 102, a subcarrier QPSK modulator 103, and a subcarrier oscillator 104.
The subcarrier QPSK modulator 103 performs QPSK modulation at a subcarrier frequency fS. Data to be subjected to the QPSK modulation is received from the application unit 105 as transmission data (TXDATA) and a transmission clock (TXCLK).
Generally, QPSK modulation requires a 90° phase shift. However, when QPSK modulation is performed by a digital circuit, the 90° phase shift can be easily created from a clock of four times fS. Also, an analog delay line may be used.
A generated QPSK modulated wave having a center frequency fS is subjected to ASK modulation by the backscatter modulator 102. The backscatter modulation can be easily performed by using a diode, a GaAs switch or the like (known). Thus, the QPSK modulated wave eventually reflected from the antenna 101 is generated in both sidebands of the frequency fO of the unmodulated carrier, that is, two bands of center frequencies fO+fS and fO−fS.
In the example shown in FIG. 5, fO+fS of the modulated wave divided into both sidebands is used. The modulated wave of fO−fS can be removed by using a band-pass filter 106 inserted between the antenna 101 and the backscatter modulator 102, for example. However, a loss from insertion of the band-pass filter 106 occurs twice, causing a decrease in reflection efficiency. In addition, the insertion of the band-pass filter 106 increases the device cost.
In the radio transmission and reception device 110, the backscattered modulated wave of fO+fS is received by the antenna 111, the circulator 112, and the receiving unit 114.
The receiving unit 114 performs direct conversion reception at the frequency fO+fS of the local oscillator 115. The QPSK modulated wave is converted into baseband signals I and Q. The baseband signals I and Q are sent to the baseband processing unit 118.
The baseband processing unit 118 performs QPSK demodulation processing (carrier synchronization and symbol synchronization), thereby generating received data RXDATA and a received clock RXCLK, and then supplies the received data RXDATA and the received clock RXCLK to the application unit 119.
However, the above-described method of shifting the unmodulated carrier of the frequency fO from the tag reader side by fS on the tag side and returning the reflected wave has the following problems.
(1) The reflected modulated wave appears in a state of being divided into both sidebands shifted by the subcarrier frequency to the plus side and the minus side from the center of the unmodulated carrier from the reader. Since the necessary modulated wave is on only one side, the other side needs to be cut off by the filter. However, when the filter is used in the backscatter system, a loss from the insertion of the filter occurs in both directions, causing a decrease in reflection efficiency. In addition, an increase in the cost of the filter is a problem.
(2) The energy of the reflected wave is divided into both sidebands. Thus, when only one side is used, the energy allocated to the unused other side constitutes a power loss, thus causing a decrease in the power of the reflected wave. For example, the power of the reflected wave may be decreased by at least 3 dB. The backscatter system using ASK causes a greater decrease in the power.
FIG. 6 shows the spectrum of the reflected wave in the RFID system shown in FIG. 5. Suppose that the backscatter modulation system is ASK. Reference numeral 200 denotes a returned component of the unmodulated carrier of the frequency fO transmitted from the radio transmission and reception device 110. Reference numeral 201 denotes the QPSK modulated wave of the center frequency fO+fS. Reference numeral 202 denotes the QPSK modulated wave of the center frequency fO−fS.
As shown in the figure, the unmodulated carrier transmitted from the radio transmission and reception device 110 is divided into the components 200, 201, and 202 and then reflected. Therefore, the modulated signal on one side has a low level. That is, the level of the originally very weak reflected wave is further lowered, which is one cause of the decrease in communication distance.